Nasdaq.com lets users create accounts and build a profile to monitor stocks and news. Nasdaq said it did not believe the flaw was used by an attacker, and no personal data was compromised.

"We responded to his concerns immediately," Nasdaq said in an email statement. "We take all information security matters seriously. We work with leading security vendors and have a trained and professional team that evaluates all credible threats across our digital assets."

Cross-site scripting is an attack on a website in which a script drawn from another site is allowed to run that shouldn't. The attack can be used to steal information or potentially cause other malicious code to run.

Kolochenko said the flaw could have been used by an attacker in several ways, including stealing users' browser histories and their cookies. It could also have been used to inject HTML into a Web page and ask for people's personal details, a request that would appear to come from Nasdaq.

In another kind of attack, Kolochenko said the XSS flaw could be used to plant a link within the Nasdaq site to a malicious website.

Kolochenko said XSS flaws are common, and he has found ones in websites belonging to the BBC, Bloomberg and the Financial Times. Those organizations acknowledged the issues, but it was often a month or so before the websites were fixed, he said.

He found the Nasdaq flaw after noticing some suspicious URLs and conducting a harmless test. At that point, he stopped probing the website and notified Nasdaq by email on their support, abuse and security addresses.

"I didn't want to take it further," he said.

Nasdaq's trading halted on Aug. 22 after a technical problem with a core data feed that distributes market data for securities listed on its exchange. A connectivity issue degraded the ability of the Securities Industry Processor (SP) system to consolidate and disseminate quote and trade information on Nasdaq listed securities.